1. 32A-4 Chromatographic Detectors
Dozens of detectors have been investigated and used with gas chromatographic separations.

2. Characteristics of Ideal Detector
The ideal detector for gas chromatography has the following characteristics:
Adequate sensitivity, in general, sensitivities of present-day detectors lie in the range of 10–8 to 10–15 g solute/s.
Good stability and reproducibility.
A linear response to solutes that extends over several orders of magnitude.

3. A temperature range from room temperature to at least 400oC.
A short response time that is independent of flow rate.
High reliability and ease of use.
Steady response toward all solutes or, alternatively, a highly predictable and selective response toward one or more classes of solutes.
Nondestructive of sample.

4. Needless to say, no current detector exhibits all of these characteristics.
Some of the more common detectors are listed in Table 32-1.
Four of the most widely used detectors are described in the paragraphs that follow.

6. Flame Ionization Detectors
The flame ionization detector (FID) is the most widely used and generally applicable detector for gas chromatography.
With a FID, such as that shown in Fig. 32-9, effluent from the column is directed into a small air/hydrogen flame.
Most organic compounds produce ions and electrons when pyrolyzed at the temperature of an air/ hydrogen flame.

7. Figure 32-9 A typical flame ionization detector
Figure 32-9 A typical flame ionization detector. (Courtesy of Agilent Technologies.)

8. These compounds are detected by monitoring the current produced by collecting the ions and electrons.
A few hundred volts applied between the burner tip and a collector electrode located above the flame serves to collect the ions and electrons.
The resulting current (10–12 mA) is then measured with a sensitive picoammeter.

9. The ionization of carbon compounds in a flame is a poorly understood process, although it is observed that the number of ions produced is roughly proportional to the number of reduced carbon atoms in the flame.
Because the flame ionization detector responds to the number of carbon atoms entering the detector per unit of time, it is a mass sensitive rather than a concentration-sensitive device.
As such, this detector has the advantage that changes in flow rate of the mobile phase have little effect on detector response.

10. Functional groups, such as carbonyl, alcohol, halogen, and amine, yield fewer ions or none at all in a flame.
In addition, the detector is insensitive toward noncombustible gases, such as H2O, CO2, SO2, and NOx.
These properties make the flame ionization detector most useful general detector for analysis of most organic samples including those that are contaminated with water & oxides of nitrogen and sulfur.

11. The FID exhibits a high sensitivity (10–13 g/s), large linear response range (107), and low noise.
It is generally rugged and easy to use.
Disadvantages of the flame ionization Detector
it destroys the sample during the combustion step and
requires additional gases and controllers.

12. Thermal Conductivity Detectors
The thermal conductivity detector (TCD), which was one of the earliest detectors for gas chromatography, still finds wide application.
This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas.
The heated element may be a fine platinum, gold, or tungsten wire or, alternatively, a small thermistor.
The electrical resistance of this element depends on the thermal conductivity of the gas.

13. Fig. 32-10a shows a cross-sectional view of one of temperature-sensitive elements in a TCD.
Four thermally sensitive resistive elements are often used.
A reference pair is located ahead of the sample injection chamber and sample pair immediately beyond the column.
Alternatively, the gas stream can be split.

14. Fig Schematic of (a) a thermal conductivity detector cell and (b) an arrangement of two sample detector cells (R2 and R3) and two reference detector cells (R1 and R4).

15. The detectors are incorporated in two arms of a simple bridge circuit, as shown in Fig b, such that the thermal conductivity of the carrier gas is canceled.
In addition, the effects of variations in temperature, pressure, and electric power are minimized.
Modulated single-filament TCDs are also available.

16. The thermal conductivities of helium and hydrogen are roughly six to ten times greater than those of most organic compounds.
Thus, even small amounts of organic species cause relatively large decreases in the thermal conductivity of the column effluent, resulting in a marked rise in the temperature of the detector.
Detection by thermal conductivity is less satisfactory with carrier gases whose conductivities closely resemble those of most sample components.

17. advantages of the TCD
its simplicity,
its large linear dynamic range (about five orders of magnitude),
its general response to both organic and inorganic species,
its nondestructive character, which permits collection of solutes after detection.
chief limitation
its relatively low sensitivity (~10-8 g/s solute/mL carrier gas).
Other detectors exceed this sensitivity by factors of 104 to 107.
The low sensitivities of TCDs often precludes their use with capillary columns where sample amounts are very small.